netfilter: ctnetlink: use helper function to calculate expect ID

Delete expectation path is missing a call to the nf_expect_get_id()
helper function to calculate the expectation ID, otherwise LSB of the
expectation object address is leaked to userspace.

Fixes: 3c791076

 ("netfilter: ctnetlink: don't use conntrack/expect object addresses as id")
Reported-by:  <zdi-disclosures@trendmicro.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>